Lithium secondary batteries, which are lightweight and have high energy density, are mainly commercialized as the power source for portable devices. Also, lithium secondary batteries are currently receiving attention as large-sized, high-output power sources (e.g., power sources for automobiles), and their active development is underway.
In lithium secondary batteries, an insulating layer is provided between the positive electrode and the negative electrode. The insulating layer has the function of electrically insulating the positive and negative electrode plates from each other while retaining an electrolyte. A resin insulating layer is widely used. The resin insulating layer easily shrinks. Thus, when a lithium secondary battery is stored in a very high temperature environment for an extended period of time, an internal short-circuit tends to occur due to a physical or direct contact between the positive electrode and the negative electrode. The prevention of an internal short-circuit is becoming an increasingly important technical problem to be solved particularly in view of the recent trend of the insulating layer becoming increasingly thinner as the capacity of lithium secondary batteries is becoming increasingly higher. Once an internal short-circuit occurs, the short-circuit further expands due to Joule's heat generated by the short-circuit current. In some cases, the battery may overheat.
It is also very important to assure safety when batteries internally short-circuit. Thus, techniques to enhance the safety of batteries under internal short-circuit conditions are being extensively developed. For example, there has been proposed a technique in which insulating tape is affixed to the exposed part of a current-collecting terminal of a positive electrode or negative electrode to prevent an internal short-circuit between the current-collecting terminals (for example, see Patent Document 1). There has also been proposed a technique in which an ion-conductive insulating layer composed of ceramic particles and a binder is printed on an electrode plate (for example, see Patent Document 2).
Further, to assure safety under an internal short-circuit condition, it is also very important to accurately evaluate the safety of a battery under an internal short-circuit condition. Battery evaluation tests for evaluating exothermic behavior under an internal short-circuit condition as a safety item of batteries such as lithium ion secondary batteries are defined, for example, by UL standards for lithium batteries (UL1642) and Standards of Battery Association of Japan (SBA G1101-1997 lithium secondary battery safety evaluation standard guidelines). These test methods are employed, for example, in patent documents to evaluate battery safety (for example, see Patent Document 3).
Conventional evaluation tests include, for example, nail penetration tests and crush tests. A nail penetration test is an internal short-circuit test which is conducted by causing a nail to penetrate through a battery from its side face or sticking a nail thereinto. When a nail is stuck, a short-circuit occurs between the positive electrode, the negative electrode, and the nail inside the battery, so that a short-circuit current flows through the short-circuited area while generating Joule's heat. Safety is evaluated by observing changes in battery temperature, battery voltage, etc., based on these phenomena. Also, a crush test is an internal short-circuit test which is performed by physically deforming a battery by using a round bar, square bar, flat plate, etc. In this manner, an internal short-circuit is caused between the positive electrode and the negative electrode, and safety is evaluated by observing changes in battery temperature, battery voltage, etc. However, conventional battery evaluation methods cannot accurately evaluate safety under internal short-circuit conditions.
Also, in considering the uses of a battery, it is necessary to understand which level of safety performance the battery has in the event of an internal short-circuit, such as “generates no heat” or “generates a little heat”. However, since conventional evaluation methods cannot accurately evaluate safety under internal short-circuit conditions, it is also very difficult to identify safety level.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-247064    Patent Document 2: Japanese Laid-Open Patent Publication No. Hei 10-106530    Patent Document 3: Japanese Laid-Open Patent Publication No. Hei 11-102729